It is known to form a microelectronic assembly by a direct chip attach method, commonly referred to as flip chip, wherein an integrated circuit die is mounted directly onto a substrate, such as a printed circuit board, by solder bump interconnections. The integrated circuit die is spaced apart from the printed circuit board by a gap, and the solder bump interconnections extend across the gap and connect bond pads on the integrated circuit die to bond pads on the printed circuit board. In this manner, the integrated circuit die is attached to the printed circuit board, and electrical signals are conducted to and from the die for processing.
Because of differences in the coefficients of thermal expansion of the die and the board, stresses are created when the assembly is subjected to thermal cycling of the type experienced during operation. These stresses tend to fatigue the solder bump interconnections and can lead to failure of the assembly. In order to strengthen the solder joints without affecting the electrical connection, the gap is filled with a polymeric encapsulant.
The encapsulant is typically applied after the die is attached by the solder bump interconnections to the printed circuit board. A curable mixture comprising inorganic particulate filler dispersed in a polymeric precursor is dispensed onto the printed circuit board adjacent to the die and is drawn into the gap by capillary action. The precursor is then cured, typically by heating, to form the encapsulant. The encapsulant strengthens the assembly and protects the solder bump interconnections from environmental damage.
The curing process, however, creates thermal stresses during the heating and cooling of the assembly. Stresses are also experienced during use of the microelectronic assembly. These stresses can be detrimental to the die and the solder bump interconnections and can cause a detrimental warping of the integrated circuit die. The stresses can also lead to cracking of the die, particularly on the side of the die opposite the solder bump interconnections, commonly referred to as the non-active face of the die.
Therefore, a need exists to reduce the detrimental effects of thermally induced stresses upon an integrated circuit die and solder bump interconnections of a microelectronic assembly. Further, a need exists to strengthen and improve the reliability of integrated circuit die that includes reducing cracks or other defects and to extend the useful life of a microelectronic assembly.